At present, a lithium secondary battery is generally employed as an electric source for driving small electronic devices. The lithium secondary battery essentially comprises a positive electrode, a non-aqueous electrolytic solution, and a negative electrode. A lithium secondary battery utilizing a positive electrode of lithium compound oxide such as LiCoO2 and a negative electrode of carbonaceous material or lithium metal is preferably used. As the electrolytic solution for the lithium secondary battery, a carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) is preferably used.
Heretofore, there have been proposals to incorporate a variety of additives to a non-electrolytic solution so as to improve various performances of a lithium secondary battery.
U.S. Pat. No. 5,709,968 (corresponding to Japanese Patent Provisional Publication No. 9-50822) describes a number of compounds including p-fluoroanisole and 2,4-difluoroanisole to be used as additives for keeping a lithium secondary battery from over-charging.
Japanese Patent Provisional Publication No. 11-329490 describes pentafluorobenzene derivatives in which the six hydrogen atoms on the benzene ring are replaced with five fluorine atoms and one substituted ester group, substituted acyl group or trifluoromethyl group, such as methyl pentafluorobenzenecarboxylate and octafluorotoluene, which are employable for improving the cycle performance of a lithium secondary battery.